Field of the Invention
This invention relates generally to a structural form, and more particularly to such a form which is constructed of sheet material which is folded to provide a relatively rigid structure. Although the structural form of the present invention has particular application as a concrete form, it is to be understood that it may be employed as any form which is intended to support weight or forces imposed thereon.
Prior Art
A variety of forms constructed of a foldable or pliable material, such as cardboard, are available for supporting a mass, such as wet concrete. Such structural forms are available in a variety of shapes for performing a specific function, such as the casting of concrete to a desired shape. In this respect, such presently known forms are limited in their ability to concrete in a variety of shapes, and are generally capable of casting concrete in only one shape. Reference is made, for example, to U.S. Pat. No. 2,296,352 in which a concrete form is disclosed for casting concrete piers. The concrete form disclosed in that patent is limited to a cylindrical shape. Other cylindrically shaped forms for casting a cylindrical aperture in concrete are disclosed in U.S. Pat. Nos. 2,659,134 and 2,694,847. It can be appreciated that the concrete forms disclosed in the above mentioned patents are limited to only one shape.
A problem which has long existed in the building construction industry is that of casting concrete columns, particularly such columns having a rectangular cross-section. It has been the practice in the past to construct forms for such columns of relatively heavy material, such as wood planks. After the concrete has been poured in such a from and has set, it is usually not possible to salvage the material of the form. Accordingly, it can be appreciated that such forms can be relatively expensive. Also, the construction and fabrication of such forms requires the use of relatively expensive labor.
With the exception of such custom-made forms, no other forms are available for casing concrete columns having a rectangular cross-sectional shape. Of the known prefabricated structural forms which are capable of casting a shape which is other than cylindrical, none are capable of casting a column.
It can be appreciated that the concrete forms dislosed in the above mentioned patents rely entirely upon the shear strength of the sheet material from which they are constructed. Such shear strength is that strength which resists a force exerted in a direction which is orthogonal to the plane of the sheet material. However, the strength of sheet material, such as cardboard, is considerably greater in a direction which is parallel to the plane thereof. This strength is called the axial strength. That is, if a force is exerted at right angles to sheet materal, it will flex, thereby providing relatively little restraint to such a force. However, if such a force is directed against an edge of the sheet material and parallel to the plane thereof, a relatively large force can be supported. Accordingly, it can be appreciated that the axial strength of sheet material, such as cardboard, is considerably greater than the shear strength thereof.
A number of prefabricated structural forms employ the axial strength of sheet material. Representative examples of such forms are disclosed in U.S. Pat. Nos. 2,101,019; 2,775,019; 3,148,444 and 3,358,960. However, the structural forms disclosed in these patents are not capable of casting a concrete column, but are limited to casting a void in a concrete slab. Accordingly, there is no known structural form presently available for casing a concrete column which employs the axial strength of sheet material to support forces imposed thereon.